Apoptosis, or programmed cell death, typically occurs in the normal development and maintenance of healthy tissues in multicellular organisms. It is a complex process which results in the removal of damaged, diseased or developmentally redundant cells, in the absence of signs of inflammation or necrosis.
Intrinsic apoptotic pathways are known to be dysregulated, most particularly in cancer and lymphoproliferative syndromes, as well as autoimmune disorders such as multiple sclerosis, in neurodegenerative diseases and in inflammation. As well, alterations in a host apoptotic response have been described in the development or maintenance of viral and bacterial infections.
The caspases are a family of proteolytic enzymes from the class of cysteine proteases which are known to initiate and execute apoptosis. In normal cells, the caspases are present as inactive zymogens, which are catalytically activated following external signals, for example those resulting from ligand driven Death Receptor activation, such as cytokines or immunological agents, or by release of mitochondrial factors, such as cytochrome C following genotoxic, chemotoxic, or radiation-induced cellular injury. The Inhibitors of Apoptosis Proteins (IAPs) constitute a family of proteins which are capable of binding to and inhibiting the caspases, thereby suppressing cellular apoptosis. Because of their central role in regulating Caspase activity, the IAPs are capable of inhibiting programmed cell death from a wide variety of triggers, which include loss of homeostatic, or endogenous cellular growth control mechanisms, as well as chemotherapeutic drugs and irradiation.
The IAPs contain one to three homologous structural domains known as baculovirus IAP repeat (BIR) domains. They may also contain a RING zinc finger domain at the C-terminus, with a capability of inducing ubiquitinylation of IAP-binding molecules via its E3 ligase function. The human IAPs, XIAP, HIAP1 (also referred to as cIAP2), and HIAP2 (cIAP1) each have three BIR domains, and a carboxy terminal RING zinc finger. Another IAP, NAIP, has three BIR domains (BIR1, BIR2 and BIR3), but no RING domain, whereas Livin, TsIAP and MLIAP have a single BIR domain and a RING domain. The X chromosome-linked inhibitor of apoptosis (XIAP) is an example of an IAP which can inhibit the initiator caspase, known as caspase-9, and the effector caspases, Caspase-3 and Caspase-7, by direct binding. It can also induce the removal of caspases through the ubiquitylation-mediated proteasome pathway via the E3 ligase activity of a RING zinc finger domain. It is via the BIR3 domain that XIAP binds to and inhibits caspase-9. The linker-BIR2 domain of XIAP inhibits the activity of caspases-3 and -7. The BIR domains have also been associated with the interactions of IAPs with tumor necrosis factor-receptor associated factor (TRAFs)-1 and -2, and to TAB1, as adaptor proteins effecting survival signaling through NFkB activation. The IAPs thus function as a direct brake on the apoptosis cascade, by preventing the action of, or inhibiting active caspases and by re-directing cellular signaling to a pro-survival mode.
Progress in the cancer field has led to a new paradigm in cancer biology wherein neoplasia may be viewed as a failure of cancer cells to execute normal pathways of apoptosis. Normal cells receive continuous feedback from their environment through various intracellular and extracellular factors, and “commit suicide” if removed from this context. This induction of apoptosis is achieved by activation of the caspase cascade. Cancer cells, however, gain the ability to overcome or bypass this apoptosis regulation and continue with inappropriate proliferation. The majority of treatments for cancer induce at least a partial apoptotic response in the cancer target cell, resulting in remission or initiation of tumor regression. In many cases, however, residual cells which are apoptosis-resistant are capable of escaping therapy and continuing the process of oncogenic/genetic change, resulting in the emergence of highly drug-resistant, metastatic disease which overcomes our ability to effectively treat the disease. Furthermore, most cancer therapies, including radiation therapy and traditional chemotherapy do induce apoptosis in cancer cells, but cause additional cellular injury, due to their lack of specificity in inducing apoptosis solely in cancer cells. The need to improve the specificity/potency of pro-apoptosis agents used to treat cancer, and indeed other proliferative disorders, is important because of the benefits in decreasing the side effects associated with administration of these agents. Therefore, finding novel means of inducing apoptosis in cancer cells is a highly desired medical need and its solution offers the possibility of entirely new treatments for cancer.
A growing body of data indicates that cancer cells may avoid apoptosis by the sustained over-expression of one or more members of the IAP family of proteins, as documented in many primary tumor biopsy samples, as well as most established cancer cell lines. Epidemiological studies have demonstrated that over-expression of the various IAPs is associated with poor clinical prognosis and survival. For XIAP this is shown in cancers as diverse as leukemia and ovarian cancer. Over expression of HIAP1 and HIAP2 resulting from the frequent chromosome amplification of the 11q21-q23 region, which encompasses both, has been observed in a variety of malignancies, including medulloblastomas, renal cell carcinomas, glioblastomas, and gastric carcinomas. (X)IAP negative regulatory molecules such as XAF, appear to be tumor suppressors, which are very frequently lost in clinical cancers. Thus, by their ability to suppress the activation and execution of the intrinsic mediators of apoptosis, the caspases, the IAPs may directly contribute to tumor progression and resistance to pharmaceutical intervention. Induction of apoptosis in cancer cells by the use of potent small molecules which bind to specific IAP domains is the subject of this invention.
We and others have demonstrated the critical importance of the individual BIR domains for affecting the antiapoptotic function of the IAPs. We have proposed that antagonists of the IAPs, which may bind to the individual BIR domains, would disrupt the antiapoptotic function of the IAPs. Indeed, individual BIRs serve as critical binding sites for the N-terminal Ser-Gly-Val-Asp, Ser-Gly-Pro-Ile and Ala-Thre-Pro-Ile residues of the Caspases 3, 7, and 9, respectively, and such binding is imperative for the Caspase-inhibitory function of the IAPs. The binding of N-terminal AxPy tetra-peptide residues to XIAP results in the release of the active caspases 3, 7 and 9. In the case of the other IAPs, such as c-IAP1 and c-IAP2, the functions of the BIRs, when ligand-bound, appear to direct the activation of the ubiquitin ligase RING function of the IAPs to a bound target, or individual IAPs themselves, to cause proteosomal loss. In either case, small molecule antagonists of the IAPs should be excellent pro-apoptotic agents, with potential uses in cancer, various proliferative disorders and inflammation.
A mammalian mitochondrial protein, namely Second Mitochondria-derived Activator of Caspases (SMAC) which antagonizes IAP function, binds mainly to the BIR 3 or 2 sites on respective IAPs via an AxPy amino-terminal tetrapeptide. Four Drosophila death-inducing proteins, Reaper, HID, Grim, and Sickle, which antagonize the ability of the Drosophila IAPs to inhibit caspases, also bind the BIR domains of the analogous Drosophila IAPs via a short AxPy amino-terminal tetrapeptide, a sequence that fits into the BIR binding pocket and disrupts IAP-caspase interactions.
The overall topology of individual BIR domains is highly conserved between the human IAPs and between individual BIR domains of the human IAPs, each BIR being a zinc finger polypeptide domain, locked into a coordinated Zn atom by two cysteines and a histidine residue. The X-ray crystallographic structures of XIAP BIR2 and BIR3 reveal a critical binding pocket for an AXPY motif on the surface of each BIR domain. There are alterations in the intervening amino acid sequences that form the binding pocket and groove in both BIR2 and BIR3. Likewise, we have described homologous domains in the BIRs of other IAPs cIAP1 and cIAP2. This opens the possibility of obtaining various classes of natural and synthetic binding compounds which will have different specificity and binding affinities between each of the BIR domains for each of the IAPs. Discerning the way in which such compounds will affect the biological function of the IAPs in cancer cells vs normal cells is a major new challenge in the discovery of novel mechanism agents to treat cancer and other proliferative disorders where dysregulated IAP function is observed. It is our finding that certain classes of BIR binding compounds may bind to IAP BIRs, with unexpected selectivity and potency, resulting in distinct therapeutic advantages for certain structural classes, potentially resulting from either IAP loss of function or loss of cellular IAP protein, or both.
A number of peptidic AxPy-like and heterocyclic modified AxPy peptidic compounds have been described which activate cellular Caspase 3 by reportedly binding to XIAP BIR3. For a recent reviews, see Elmore et al., Annual Reports in Medicinal Chemistry, 40 (2006) 245-262; Sun et al., Bioorg. Med. Chem. Let. 15 (2005) 793-797; Oost et al., J. Med. Chem., 2004, 47(18), 4417-4426; Park et al., Bioorg. Med. Chem. Lett. 15 (2005) 771-775; Franklin et al., Biochemistry, Vol. 42, No. 27, 2003, 8223-8231; Kip et al., Biochemistry 2002, 41, 7344-7349; Wu et al., Chemistry and Biology, Vol. 10, 759-767 (2003); Glover et al., Analytical Biochemistry, 320 (2003) 157-169; United States published patent application number 20020177557; and United States published patent application number 20040180828; United States published patent application number US2006/0025347A1; United States published patent application number US2005/0197403A1; and United States published patent application number US2006/0194741A1.
The aforesaid compounds have been shown to target an isolated BIR3 domain of XIAP via displacement of a fluorescently-labeled probe and they appear to induce an apoptotic event in a select set of cancer cell lines with potency in the low micromolar-nanomolar range. These compounds displayed poor in-vivo activity, likely due to limited bioavailability and may therefore have limited therapeutic application.
Thus, IAP BIR domains represent an attractive target for the discovery and development of novel therapeutic agents, especially for the treatment of proliferative disorders such as cancer.